A magnetic resonance imaging apparatus is an apparatus that images an inside of a subject by using a magnetic resonance phenomenon. Such magnetic resonance imaging apparatus includes a static magnetic-field magnet that generates a static magnetic field in a scan zone, a transmitting coil that applies a radio-frequency magnetic field to a subject placed in the static magnetic field, and a receiving coil that receives a magnetic resonance signal emitted from the subject owing to an application of a radio-frequency magnetic field.
In a magnetic resonance imaging apparatus, the receiving coil is generally arranged on the inner side of the transmitting coil. For this reason, when a radio-frequency magnetic field and an electric field are applied to a subject by the transmitting coil, the radio-frequency magnetic field and the electric field are also applied to the receiving coil, as a result, a current and a voltage are induced in the receiving coil. In this way, when a current and a voltage are induced in the receiving coil, a loss of electric power supplied by the transmitting coil may be produced; a spatial distribution of the radio-frequency magnetic field may become non-uniform; the receiving coil may generate heat; and/or a part of the receiving coil may be broken, in some cases.
Therefore, aiming to suppress a current and a voltage induced in a receiving coil, a magnetic resonance imaging apparatus sometimes includes a BALanced UNbalanced (BALUN) transformer in some cases (for example, see JP-A H7-136145).
However, the conventional technology described above has a possibility that when a strong current and a strong voltage beyond an assumption are induced in the receiving coil, the safety of imaging may decrease, as explained below.
Specifically, when a strong current and a strong voltage beyond an assumption are induced in the receiving coil owing to an application of a radio-frequency magnetic field and an electric field by the transmitting coil, the BALUN sometimes generates heat with the current and the voltage in some cases. When the BALUN overheats, there is a possibility that the BALUN may be broken, or a subject may suffer a burn due to heat generation of the BALUN. This is not limited to BALUNs, and similarly occurs in other electronic circuits connected to a receiving coil, for example, a trap circuit that switches between drive and stop of the receiving coil.